This invention relates to the field of photonic crystal waveguides and systems using photonic crystal waveguides.
Waveguides play important roles in numerous industries. For example, optical waveguides are widely used in telecommunications networks, where fiber waveguides such as optical fibers are used to carry information between different locations as optical signals. Such waveguides substantially confine the optical signals to propagation along a preferred path or paths. Other applications of optical waveguides include imaging applications, such as in an endoscope, and in optical detection. Optical waveguides can also be used to guide laser radiation (e.g., high intensity laser radiation) from a source to a target in medical (e.g., eye surgery) and manufacturing (e.g., laser machining and forming) applications.
The most prevalent type of fiber waveguide is an optical fiber, which utilizes index guiding to confine an optical signal to a preferred path. Such fibers include a core region extending along a waveguide axis and a cladding region surrounding the core about the waveguide axis and having a refractive index less than that of the core region. Because of the index-contrast, optical rays propagating substantially along the waveguide axis in the higher-index core can undergo total internal reflection (TIR) from the core-cladding interface. As a result, the optical fiber guides one or more modes of electromagnetic (EM) radiation to propagate in the core along the waveguide axis. The number of such guided modes increases with core diameter. Notably, the index-guiding mechanism precludes the presence of any cladding modes lying below the lowest-frequency guided mode for a given wavevector parallel to the waveguide axis. Almost all index-guided optical fibers in use commercially are silica-based in which one or both of the core and cladding are doped with impurities to produce the index contrast and generate the core-cladding interface. For example, commonly used silica optical fibers have indices of about 1.45 and index contrasts ranging from about 0.2% to 3% for wavelengths in the range of 1.5 mm, depending on the application.
Another type of waveguide fiber, one that is not based on TIR index-guiding, is a Bragg fiber, which includes multiple alternating dielectric layers surrounding a core about a waveguide axis. The multiple layers form a cylindrical mirror that confines light to the core over a range of frequencies. The alternating layers are analogous to the alternating layers of a planar dielectric stack reflector (which is also known as a Bragg mirror). The multiple layers form what is known as a photonic crystal, and the Bragg fiber is an example of a photonic crystal fiber. Photonic crystal structures are described generally in Photonic Crystals by John D. Joannopoulos et al. (Princeton University Press, Princeton N.J., 1995).
Drawing a fiber from a preform is the most commonly used method for making fiber waveguides. A preform is a short rod (e.g., 10 to 20 inches long) having the precise form and composition of the desired fiber. The diameter of the preform, however, is much larger than the fiber diameter (e.g., 100's to 1000's of times larger). Typically, when drawing an optical fiber, the material composition of a preform includes a single glass having varying levels of one or more dopants provided in the preform core to increase the core's refractive index relative to the cladding refractive index. This ensures that the material forming the core and cladding are rheologically and chemically similar to be drawn, while still providing sufficient index contrast to support guided modes in the core. To form the fiber from the preform a furnace heats the preform to a temperature at which the glass viscosity is sufficiently low (e.g., less than 108 Poise) to draw fiber from the preform. Upon drawing, the preform necks down to a fiber that has the same cross-sectional composition and structure as the preform. The diameter of the fiber is determined by the specific rheological properties of the fiber and the rate at which it is drawn.
Preforms can be made using many techniques known to those skilled in the art, including modified chemical vapor deposition (MCVD), outside vapor deposition (OVD), plasma activated chemical vapor deposition (PCVD) and vapor axial deposition (VAD). Each process typically involves depositing layers of vaporized raw materials onto a wall of a pre-made tube or rod in the form of soot. Each soot layer is fused shortly after deposition. This results in a preform tube that is subsequently collapsed into a solid rod, over jacketed, and then drawn into fiber.
Optical fibers applications can be limited by wavelength and signal power. Preferably, fibers should be formed from materials that have low absorption of energy at guided wavelengths and should have minimal defects. Where absorption is high, it can reduce signal strength to levels indistinguishable from noise for transmission over long fibers. Even for relatively low absorption materials, absorption by the core and/or cladding heats the fiber. Defects can scatter guided radiation out of the core, which can also lead to heating of the fiber. Above a certain power density, this heating can irreparably damage the fiber. Accordingly, many applications that utilize high power radiation sources use apparatus other than optical fibers to guide the radiation from the source to its destination.